ultrasound 1

Question 1

higher frequency

Answer 1

• shorter wavelength
• shorter penetration
• good resolution

Question 2

wave speeds in order

Answer 2

air = 330 m/s
fat = 1450 m/s
soft tissue = 1540 m/s
muscle = 1580 m/s
bone = 3000 m/s

Question 3

what is sound

Answer 3

sound is a mechanical, longitudinal wave that travels in a straight line
- produced when an object vibrates

Question 4

what are vibrations transmitted through

Answer 4

a medium (air, water, tissue) between adjacent particles of surrounding medium

Question 5

what is frequency defined as

Answer 5

number of vibrations per second

Question 6

advantages of sound waves

Answer 6

• non-iosnising radiation
• non-invasive
• acceptable to most patients
• real-time imaging
• inexpensive
• mobile
• images can differentiate between soft tissues

Question 7

disadvantages to sound waves

Answer 7

• operator dependent
• limited use around bone and air
• limited by patient habits
• different to distinguish malignant and benign
• no functional information

Question 8

what are piezo-electric materials

Answer 8

they are commonly used in ultrasound transducers and they emit and receive US waves

Question 9

when is a piezo-electric crystal/ ceramic usd

Answer 9

in ultrasound generation

Question 10

what happens when a piezo-electric crystal is used

Answer 10

when an alternating current is applied, it undergoes rapid extensions and contractions which causes the material to deform

Question 11

what does mechanical deformation lead to

Answer 11

the mechanical deformation of the piezo-electric material when an alternating current is applied generates US waves in the form of pressure waves

Question 12

what happens in the first step of receiver mode?

Answer 12

the US transducer detects and converts incoming US waves into electrical signals

Question 13

what happens in the second step of the receiver mode

Answer 13

incoming waves strike crystal and exert mechanical stress causing deformation which generates a small electrical current (piezo-electric effect)

Question 14

what happens when a small EC is generated from deformation

Answer 14

• EC is detected and converted to electrical signal by transducer

Question 15

what happens to the electrical signal produced by the transducer

Answer 15

• electrical signal is processed by US system to create an image of internal structures
• based on time delay and amplitude of received doses

Question 16

what is the pulse echo technique

Answer 16

• crystal cannot transmit and receive sound waves at the same time so…
• transducer emits short burst of US waves directed into the body
• pulse travels through medium and different internal materials
• when US waves interferes with a material of different acoustic impedance the portion of US energy is reflected back to the transducer
• the same US transducer that emitted the pulse is now the reciever

Question 17

advantages of pulse echo technique

Answer 17

• better echoes with no interference between transmitted and reflected sound waves
• alternate generation/ detection only requires 1 crystal
• possible to measure distances + dimensions of image

Question 18

disadvantages of the pulse echo technique

Answer 18

• limited resolution
• attenuation of signals
• dependency on tissue characteristics
• reflection artefacts
• limited penetration in dense materials
• limited contrast resolution

Question 19

what is the coupling medium

Answer 19

• US waves reflect strongly where air meets biological tissue
• coupling medium used to facilitate transmission of US waves between transducer and patients skin

Question 20

what is the purpose of a coupling medium

Answer 20

• eliminate air gaps between transducer and skin
• can be gel, gel pads, oil based gel etc

Question 21

frame rate needs to be …

Answer 21

fast enough to see real time image

Question 22

what is acoustic impedance

Answer 22

• property of medium that describes ability to transmit sound waves and measure how much a material resists transmission of acoustic energy

Question 23

how is acoustic impedance defined as

Answer 23

z = acoustic impedance
p = density
c = speed of sound
this describes the resistance of a material to the transmission of sound waves
z = p . c